The present invention pertains to the art of continuous casting of metals, and more specifically to a new and improved method and apparatus for horizontal continuous casting, wherein molten metal is fed through a nozzle of ceramic or refractory metal into a stationary, liquid-cooled mold to form a continuous wire, or rod, that is drawn out of the exit end of the mold in incremental steps by a pair of pinch rolls. The apparatus is particularly adapted for producing welding consumables in cobalt, nickle, and iron base alloys, and is presently designed for small sections of 3 mm to 8 mm cross-sectional dimensions, although not limited to such sizes.
A continuous caster of the type described above has been manufactured and sold by Steel Casting Engineering, Ltd., of Orange, Calif., for several years, and has enjoyed considerable commercial success. However, in the course of on-going development to improve and perfect the machine, it has been observed that the surface of the cast wire, or rod, is slightly discolored at regular intervals along its length, apparently due to oxidation, and that oxides formed within the casting machine were tending to damage the ceramic nozzle and to erode the walls of the liquid-cooled mold, so that the life of the mold was shortened through loss of dimensional integrity. Moreover, if the refractory metal of the nozzle forms an oxide which can be absorbed by the metal being cast, the chemistry of the metal can be changed enough to cause the product to be out of specification.
The discolorations on the cast wire, or rod, appeared to have been formed within the casting machine at the location where the nozzle joins the liquid-cooled mold, as each individual discoloration was closely adjacent one of the almost imperceptible casting marks produced by the step-by-step advancement of the wire. The puzzling aspect of the problem was that the metal is not exposed to the atmosphere at this point, and therefore there was no apparent source of oxygen to produce this oxidation. While the ceramic nozzle is very slightly porous, it did not appear to be sufficiently porous to account for the amount of oxidation that was occurring.
The only remaining possibility was that, somehow, oxygen was getting in to the hot metal through the interface between the nozzle and the liquid-cooled mold. The ceramic nozzle is mechanically clamped against the end of the mold, and is a tight fit; hence, there seemed to be little likelihood of air leaking through at this point. However, there is an extreme temperature differential between the nozzle, which is immersed at one end in a molten metal and has a constant stream of molten metal flowing through its center, and the liquid-cooled mold, which is less than 200.degree. F. The temperature difference can thus range up to 3000.degree. or more.
It was noted that the oxidation problem was more pronounced on very humid days, and it became evident that the problem was caused, at least in part, by moisture in the atmosphere. Whether the water vapor in the air was being dissociated by the extreme temperature differential at the interface, or served as a catalyst in the other chemical reactions that were taking place is a matter of speculation; the fact remains that a minute quantity of moist air was penetrating through the slightly porous ceramic nozzle and/or the interface, to produce oxidation of the cast metal, despite every effort to prevent it.